Recently when working with a very old, decrepit server that was running out of disk space I went to compress a folder of logs in NTFS and the compress box was greyed out and I was unable to compress it.

This is because in NTFS the compression attribute isn’t supported for volumes with cluster sizes larger than 4k — this happened to be sized at 64k thus the inability.

The way that I typically check this is by simply running “CHKDSK D:” replacing D: with your prospective drive letter. The output of this will display your cluster size in bytes (divide by 1024 to get the cluster size in kb)